The present invention is generally directed to an implantable cardiac stimulation device and is particularly directed to a method for altering the treatment of a patient""s heart in response to the detection of ischemia in the patient""s cardiac tissue.
Ischemia is a condition resulting from insufficient blood flow through the heart muscle. The reason therefore is blocking or passage congestion of coronary blood vessels of the heart. Blood penetration of the heart muscle is possible only in the diastolic phase, that is the phase between two consecutive contractions of the heart, when the aortic valve is closed. About 60% of the oxygen content inside the heart tissue is consumed during a heart contraction and in order to maintain the pumping efficiency of the heart, the consumed oxygen must be refilled until the next contraction.
An increased heart rate results in only minor shortening of the systolic phase, that is the contraction phase of the heart, and consequently an increased heart rate results mainly in a shortening of the diastolic phase, the period during which oxygen is supplied to the heart as mentioned above. An increased workload will consequently worsen the situation for an ischemic patient.
A large portion of cardiac ischemia is silent. It has been suggested that up to 80% of ischemic heart diseases are silent, i.e., a state of ischemia which the patient is not aware of. In other cases, a symptomatic (episodic) ischemia, that is angina pectoris, heart insufficiency or infarct, will force the patient, because of the associated pain, to stillness, with a reduced heart rate as a consequence.
Some patients experience a condition referred to as chronotropic incompetence in which their natural pacemaker, the sinoatrial (SA) node, is incapable of automatically adjusting the heart""s pacing rate in response to increased metabolic demands, e.g., exercise. To accommodate this condition, rate modulated pacing systems have been developed which monitor the patient""s activity level using an activity or physiologic sensor, e.g., an accelerometer, minute ventilation sensor or the like, to adjust the pacing rate in response to the detected activity level. In such rate modulated pacing systems, a programmable value sets the highest pacing rate that can be achieved in response to the sensor input, the so-called maximum sensor rate (MSR). When this rate modulation capability is provided to a patient experiencing ischemia, the sensor can often drive the pacing rate to a rate which exacerbates the current ischemia and may further damage the patient""s heart.
It has also been proposed to provide heart stimulators provided with an ischemia detector to lower the actual stimulation rate to a base level (or below) in response to the detection of an ischemic episode, in order to slow down or stop the further development of the ischemia (see Swedish Patent Application SE 9700182-0, filed Jan. 23, 1997, or corresponding PCT application PCT/SE98/0043, filed Jan. 13, 1998).
In a dual-chamber sensing and tracking pacing system, the ventricle is stimulated in response to detected atrial activity, normally a programmable value sets the highest allowable ventricular pacing rate, the so-called maximum tracking rate (MTR). In some cases, the maximum sensor rate and the maximum tracking rate may be set so high that in an ischemic situation a prolonged high pacing rate can stimulate the heart to an infarct.
U.S. Pat. No. 6,021,350 has partially addressed this issue by reducing the MSR/MTR to a prescribed lower rate, e.g., 100-120 beats per minute (bpm), in response to a detected ischemia but at the cost of limiting the rate responsive capability of the heart stimulation device.
The present invention is directed to further improvements of the therapies delivered in response to a detected ischemia that avoid the adverse effects of ischemia while preserving the advantages of rate modulated pacing.
The present invention is directed to an apparatus and method for monitoring the patient""s heart for the presence of ischemia and altering the treatment delivered by an implantable cardiac stimulation device to minimize adverse ischemic effects, e.g., angina pectoris. Embodiments of the present invention are particularly beneficial when used with a rate responsive cardiac stimulation device. Typically, a rate responsive cardiac stimulation device increases its pacing rate (up to a maximum sensor rate) in response to increases in the patient""s activity level. The rate of this change is referred to in this patent as the aggressiveness of the rate responsiveness. However, in an ischemic state, the aggressiveness of the rate responsiveness may cause the heart to pace at a rate that exacerbates the ischemic effects. Accordingly, embodiments of the present invention alter the pacing regimen in one or more of the following ways. First, the maximum sensor rate is adaptively decremented in response to a detected ischemic state. Second, the aggressiveness of the rate responsiveness is adaptively decreased in response to a detected ischemic state. Third, the atrio-ventricular delay (AV) delay is adaptively extended in response to a detected ischemic state. Conversely, when an ischemic state is no longer detected, the adapted variable is incrementally returned toward its original value. Accordingly, ischemia can be minimized while still maintaining the rate responsive features of the implantable cardiac stimulation device.
A preferred implantable cardiac stimulation device operates according to a set of prescribed adaptable pacing parameters including a pacing rate set as a function of the patient""s activity level up to a maximum sensor rate and an aggressiveness value specifying the rate of increase of the pacing rate per the increase in the patient""s activity level, wherein each of the parameters has an original value in the absence of ischemia. A preferred device monitors for the presence of ischemia in the patient""s heart and, once detected, incrementally alters at least one of the prescribed pacing parameters of the cardiac stimulation device by a specified amount from its respective original value.
In a further aspect of a preferred invention, the device continues to monitor for the presence of ischemia and continues to periodically incrementally alter at least one of the prescribed pacing parameters by the defined amount while ischemia is still detected. However, after a prescribed time period if the ischemia is absent, the device incrementally adjusts the altered pacing parameter back towards its original value.
In a still further aspect of a preferred invention, the prescribed adaptable pacing parameters are only altered within a defined range of increments.
Preferred embodiments include the maximum sensor rate (MSR), the aggressiveness and the AV/PV delay values within the set of prescribed adaptable pacing parameters. When ischemia is present, one or more of the following may occur: the MSR is decreased, the aggressiveness is decreased, and the AV/PV delay values are increased. Conversely, after a prescribed time period without ischemia, one or more of the following may occur: the MSR is increased, the aggressiveness is increased, and the AV/PV delay values are decreased.
The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.